Lightship

noun

1. A ship with a powerful light or warning signals that is anchored in dangerous waters to alert other vessels.
2. A vessel riding at anchor and displaying a light for the guidance of mariners, in a position where the bottom or the depth would render a fixed lighthouse-structure impracticable.
3. A vessel equipped like a lighthouse, carrying at the masthead a brilliant light, and moored off a shoal or place of dangerous navigation where a permanent lighthouse would be impracticable, to serve as a guide for mariners.

   "the Ambrose lightship off New York was rammed and damaged in 1950 by the Santa Monica"

4. A ship equipped with a very large lamp; it can be positioned to warn off other ships from dangerous locations, as a sort of portable lighthouse.
5. a ship equipped like a lighthouse and anchored where a permanent lighthouse would be impracticable